1. Field of the Invention
This invention relates in general to electrical oscillators and in particular to oscillators used as transmitters and having a radiating element such as an antenna.
2. Description of the Prior Art
Three basic oscillator circuits have been used in garage door transmitter circuits of the prior art and such circuits without considering the DC connections and auxiliary components require an active device, a tuned circuit and a feedback circuit. A circuit ground point where the AC potential with respect to an earth ground is zero generally exists at one of the electrodes of the active element such as a transistor. With small remote control unshielded battery operated transmitters having no external connection, a concept of a ground may be nebulous, however, this concept becomes more obvious in a balanced circuit such as a circuit connected in push-pull with a pair of active elements or in the case of a shielded circuit. Where no actual ground exists in a transmitter or oscillating circuit, a "common" point or element of the active device can be defined.
FIG. 1a illustrates a Colpitts oscillator. FIG. 1b illustrates a Hartley oscillator circuit and FIG. 1c illustrates a push-pull oscillator circuit. It is obvious in the Hartley oscillator circuit of FIG. 1b that the mid-point of the coil is at AC ground potential. There is a virtual ground at the center of the coil in the circuit of FIG. 1c as well as in the circuit of FIG. 1a where it is a point determined by the ratio of the two capacitors in the resonating circuit.
Garage door transmitters operate in the frequency range such as 390 MHz and in this range, a tuning inductor can be constructed as a relatively large single turn loop illustrated in FIG. 2 on a substrate 10. The center of the loop is grounded as illustrated and a loop is formed as a wide flat strip so as to minimize series resistance and inductance per unit length of conductor and the efficiency of the loop as a radiator increases as the area increases.
Unfortunately, the circuit illustrated in FIG. 2 is subject to pulling of its frequency from at least two separate sources. If an ungrounded conductive sheet is placed near and parallel to the loop, the frequency will rise due to loss of inductance caused by inductive coupling to the conductive sheet. On the other hand, if a metallic object is effectively grounded and is coupled capacitively to the "hot" end of the coil, the frequency will be lowered because of the increase in tuning capacitance. In certain cases, the two effects will tend to cancel each other but in general the frequency shift will be unacceptably large. The reason for this is that garage door transmitters for example must be precisely tuned so as to cause a particular receiver for a particular garage door to respond. Frequency drift would cause other receivers not tuned to the particular transmitter to respond and also would prevent the desired receiver from responding to the transmitter.